The separation and concentration of gases using a gas separation membrane is a method that demonstrates superior energy efficiency, energy conservation and safety in the case of comparing with other methods such as distillation or high-pressure adsorption. Pioneering examples of the practical use thereof include separation and concentration of gas using a gas separation membrane and hydrogen separation in the ammonia production process. More recently, extensive studies are being conducted on a method for recovering carbon dioxide, which is a greenhouse gas, from synthesis gas or natural gas using a gas separation membrane (see, for example, Patent Documents 1, 2, and 3).
The typical form of a gas separation membrane consists of a separation layer formed on the surface of a porous support. This form is effective for providing high gas permeation volume while imparting a certain degree of strength to the membrane. The separation layer in this case refers to a layer composed only of a gas-separating polymer.
In general, the performance of a gas separation membrane is expressed using permeation rate and separation factors as indices. Permeation rate is expressed by the formula indicated below.(Permeability coefficient of gas-separating polymer)/(thickness of separation layer)As is clear from the aforementioned formula, it is necessary to reduce the thickness of the separation layer as much as possible in order to obtain a membrane having a high permeation rate. Separation factor is expressed as the ratio of the permeation rates of the two types of gases to be separated, and is dependent on the material of the gas-separating polymer.
On the basis of the above, it is necessary to reduce the thickness of the separation layer as much as possible without creating any defects in order to obtain practical performance as a gas separation membrane, and extensive studies have been conducted thereon (see, for example, Patent Documents 4 and 5). As is also clear from the aforementioned formula, permeation rate increases the higher the permeability coefficient of the gas. Namely, it is important to make a material having a large permeability coefficient and separation factor as thin as possible. This is because a gas separation membrane ideally becomes better the higher the permeability coefficient and separation factor, thereby resulting in an efficient membrane process. Separation factor is expressed as the ratio of the permeation rates of the two types of gases to be separated, and is dependent on the material of the gas-separating polymer.